1. Field of the Invention
The present invention relates to an electrically-conductive resin composition for a porous fuel cell bipolar plate and a process for the production thereof.
2. Description of the Related Art
Fuel cells are devices which, when supplied with a fuel such as hydrogen and with atmospheric oxygen, cause the fuel and oxygen to react electrochemically, producing. water and thus directly generating electricity. Because fuel cells are capable of achieving a high fuel-to-energy conversion efficiency and are environmentally adaptable, they are being developed for a variety of applications, including small-scale local power generation, household power generation, simple power supplies for isolated facilities such as campgrounds, mobile power supplies such as for automobiles and small boats, and power supplies for satellites and space development.
Such fuel cells, and particularly solid polymer fuel cells, are built in the form of modules composed of a stack of at least several tens of unit cells. Each unit cell has a pair of plate-like bipolar plates with ribs on either side thereof that define a plurality of channels for the flow of gases such as hydrogen and oxygen. Disposed between the pair of bipolar plates in the unit cell are a solid polymer electrolyte membrane and gas diffusing electrodes made of carbon paper.
This fuel cell bipolar plate is adapted to render the various single cell units electrically conductive, as well as to provide channels for the fuel and air (oxygen) supplied into the single cell units. The fuel cell bipolar plate also acts as a separation wall. To this end, the bipolar plate is required to meet various properties such as a high electrical conductivity, a high level of impermeability of gases, (electrical) chemical stability and hydrophilicity.
The fuel cell bipolar plate has heretofore been produced by a method such as one that has a step of cutting a porous carbonized carbon to form a groove, a method which has steps of subjecting a slurried mixture of graphite powder, binder resin and cellulose fiber to paper making, and then graphitizing the paper (Patent Reference 1: U.S. Pat. No. 6,187,466).
Moreover, such a porous bipolar plate leaves something to be desired in terms of strength. Thus, a bipolar plate has also been used that has an enhanced strength attained by embedding pores so as to form a dense portion. A method of embedding pores is known which has steps of subjecting a bipolar plate obtained by high pressure molding to an amount of a binder that is less than that theoretically required, and impregnating gaps with an impregnant so as to form a dense portion (Patent Reference 2: JP-A-11-195422).
Among methods of reducing the pores in the bipolar plate, a method is known which has a step of providing graphite coated by a resin having a low rate of weight reduction (Patent Reference 3: JP-A-2003-297382).
On the other hand, a bipolar plate is known that is obtained by molding (compression molding) a mixture of graphite and resin. Such a bipolar plate obtained by molding has heretofore been used in a dense form so as to prevent leakages of gas.
However, such a dense bipolar plate is susceptible to flooding in the gas flow channel (blocking of grooves by the water produced). In an attempt to solve the problem, a technique has been developed for rendering the interior of the molded bipolar plate porous (Patent Reference 4: Austrian Patent 389,020).
The porous bipolar plate absorbs the water produced by means of its porous portion. The water produced acts to cap the porous portion. Therefore, even the porous material can prevent leakages of gases.
However, the porous bipolar plate has deficiencies in terms of strength because of its porosity and has disadvantage that on occasions breakages and cracks have occurred during assembly and stacking operations.
A technique has accordingly been developed for making compact the portion of the fuel cell bipolar plate that is subjected to pressure during assembly and stacking, and at the same time for rendering the gas flow channel porous (Patent Reference 5: JP-A-2004-79194).
One of the most important requirements for a porous bipolar plate is to enhance water absorption. In other words, in order to prevent the kind of flooding mentioned above, it is necessary that water be absorbed in an amount that is more than a predetermined value. To this end, a porous fuel cell bipolar plate having an appropriate proper porosity has been demanded.